Taps - Definitions
Taps are available in countless variants, with each version being designed for a special cutting application or a group of similar cutting applications. In order to find the right tap for an application, a few basic terms should be known.
Naming the geometries
Every tap has lengths, angles and diameters that have been carefully defined in order to achieve very specific results. The naming of these geometries is regulated in DIN 2244 and is identical for every type of thread.
Particularly important parameters are
- The nominal thread diameter: the largest diameter on the thread and a frequent component of thread designations.
- The lead length: decisive for the type of hole and material for which the tap can be used.
- The rake angle: influences the chip formation and the cutting performance of the tap.
Explanation of symbols
d1 = Nominal thread diameter
d2 = Shank diameter
d3 = Gate diameter
d4 = Neck diameter
d5 = Square diameter
d6 = Neck diameter
d7 = Core diameter
l1 = Total length
l2 = Thread length
l3 = Usable thread length
l4 = Lead length
l5 = Square length
l6 = Slot length
□ = Square dimension
Z = Number of grooves
v = Lowered guide thread
xr = Chamfer angle
Nb = Groove width
m = Web width
ha = Chamfer relief
hf = Flank relief
hr = Chamfer relief
αp = Clearance angle
γp = Rake angle
Centering
Centring on the tap can be divided into three basic designs on the cutting part and on the shank. Which centering is used depends on the thread diameter and, to a lesser extent, on the lead shape.
A non-recessed centering point is also referred to as a solid point. The transition from the cutting part to the center point is smooth, as is the transition on the shank. The full point is the preferred centering type for thread sizes under 5.6 mm diameter, whereby a stepped point can also be used for cutting forms A, C, D and E from 4.2 mm diameter.
With the stepped centering tip, there is a step in front of the tip and a centering chamfer on the shank. This type of centering is often used for thread sizes between 5.6 mm and 12.8 mm in diameter.
For thread sizes over 12.8 mm in diameter, a centering hole is used on the cutting part and on the shank regardless of the chamfer shape.
Production dimensions
The dimensions for the production of taps are defined in various DIN standards and specify the lengths and tolerances for the tool as well as the shank shape. A distinction is made here between the reinforced shank and the overflow shank. The overrunning shank is smaller than the core diameter of the thread and has the same unchanged diameter over its entire length. In contrast, the diameter of the reinforced shank is larger than the thread size. This offers increased stability with hard and difficult-to-machine materials, but also has an effect on the possible thread depth that can be achieved. As the reinforced shank is thicker than the core hole for the thread, threads in deep holes cannot be realized with these tools.
Tapping grooves
The grooves on the tap are used to remove chips and distribute lubricant. The type of flutes used depends on the type of hole, the lead shape and the material.
Straight flutes can only hold a small amount of chips and are particularly suitable for short through-holes. In combination with a skiving cut, through holes in medium and long-chipping materials can also be machined well. The chip is rolled tightly through the skiving cut and transported away in the cutting direction.
Left-hand helical flutes also remove the chip in the cutting direction and are used for through holes. Right-hand helical flutes, on the other hand, remove the chip in the opposite direction to the cutting direction and are therefore particularly suitable for cutting blind holes. For particularly deep blind holes, the flutes should also be particularly strongly twisted.
Form A, C, E
- straight grooved
- for through and blind holes
The grooves can only hold part of the chips. There is hardly any chip transport in the cutting direction. Deep threads should therefore not be cut.
Form B
- straight grooved with peel cut
- for through holes
Thanks to the peel cut, the chips are removed tightly rolled in the cutting direction and chip jams are prevented. Coolant can flow in without any problems.
Form C, D
- 8 - 15° left-hand twisted spiral grooves
- for through holes
Thanks to the left-hand helical flutes, the rake angle is almost constant and produces stable gating teeth for cutting threads in high-strength materials.
The left-hand helix pushes the chips in the cutting direction.
Form C, E
- 10 - 15° right-hand helical grooves
- for blind holes
Particularly suitable for automatic lathes and multi-spindle machines. The opposing chip evacuation enables reliable thread cutting even with cross holes.
Form C, E
- 35 - 50° right-hand helical grooves
- for blind holes
The strongly twisted spiral grooves ensure that chips are reliably removed even in deep, long-chipping blind holes.
